1. Field of the Invention
The present invention relates to color printers or copiers which perform a plurality of color separations and, more particularly, to color printers which perform a plurality of color separations in which registration of the color separations is ensured.
2. DESCRIPTION OF THE RELATED ART
In color printing, successive images corresponding to each of a plurality of colors, i.e., color separations, are formed. Each single color separation is transferred to a copy sheet in superimposed registration with any prior single color image. The registered relationship of the single color images creates a multi-layered image on the copy sheet Thereafter, the multi-layered image is permanently affixed to the copy sheet, thus creating a color image.
The achievement of good copy quality using a color printer is largely dependent on the attainment of good relative registration of the various color separations forming the image. Otherwise, blurred images will result.
Color printers generally incorporate a plurality of subsystems. Examples of these subsystems are receptor loops for handling latent images, receptor loops for handling developed images, image development subsystems and image transfer subsystems. Since a plurality of subsystems cooperate to form each color separation of a single image, it is particularly difficult to establish with adequate precision the spacial position of the color separations at any given instant of time and, particularly, when color separation registration is performed.
One factor contributing to this difficulty in establishing precise color separation registration is the non-perfect cylindricity of various components of the subsystems. Another contributing factor is the eccentric location of rotating shafts about which roller components of the color printer rotate, thus causing roll run out which induces a wobble-like roller rotation Accordingly, even when components of the subsystems are driven at a perfectly constant rotational speed, the linear motion will not necessarily be uniform throughout these subsystems.
The related art has proposed techniques for improving the relative registration of various color separations.
U.S. Pat. No. 4,712,906 to Bothner et al discloses a color printer which reproduces a multi-color image and improves registration between the formation of each image. The improvement in registration is attained by utilizing a transfer drum which has a circumference substantially equal to a distance on an image member between comparable points in consecutive images. The apparatus does not disclose synchronization throughout the components of the printer.
U.S. Pat. No. 4,652,115 to Palm et al discloses a color printer which maintains precise registration of color composite images. Registration is maintained by making the length of a belt carrying an intermediate transfer medium an integer submultiple of another belt which carries a photoreceptor. A digital electronic controller synchronizes the mechanical movements of the two belts to assure proper registration of the color component images when forming a composite image. The apparatus thus requires an additional controller to ensure registration.
U.S. Pat. No. 4,847,660 to Wheatley, Jr. et al discloses a method and apparatus for registration control. Registration and synchronization between a photoreceptor web and an image receiving web is maintained by separate motors which drive the webs under the control of independent servo mechanisms. Each of the servo mechanisms is controlled to drive the respective web such that the webs have constant rotational periods. The relationship between the constant rotational periods is equal to the relationship between the lengths of the webs. Since the photoreceptor and image receiving webs have a ratio of lengths nominally equal to an integer, the relationship between the constant rotational periods is nominally an integer. The device, however, does not ensure uniform motion throughout the individual components of the printer.
U.S. Pat. No. 4,788,572 to Slayton et al discloses a dual belt synchronization system for a color printer which provides enhanced image registration. Each belt is independently driven by a DC electric motor. A digital electronic controller synchronizes the mechanical movements of the two belts to assure proper registration of color images when forming a composite image. A length of one of the belts is nominally an integer submultiple of the length of the other belt. The system achieves proper registration by synchronizing angular velocities around rollers. The synchronization of angular velocity, does not, however, create uniform motion in all subsystems of the printer. Further, the system requires an additional controller.
U.S. Pat. No. 4,965,597 to Ohigashi et al discloses a color image recording apparatus which forms a composite color image on a recording medium. The apparatus includes registration marks formed on the recording medium at equal pitches as the recording medium is transported through an image formation device. The apparatus measures each pitch between registration marks and compensates, for each interval between the registration marks, for a discrepancy between the measured pitch and the number of lines assumed when the registration marks were formed. The recording medium does not provide synchronous motion of all printer subsystem components.
U.S. Pat. No. 4,933,727 to Mizuma et al discloses a color recording apparatus comprising a timing control system which controls the timing of exposing an optical image to a photoconductor belt. The timing is synchronized with rotation of a drive roller of the photoconductor belt so that a cycle of an exposure operation is equal to a value of one cycle of the driver roller multiplied by a number of rotation. The device thus only controls exposure timing in accordance with rotation of a single drive roller of the photoconductor belt.
U.S. Pat. No. 4,961,089 to Jamzadeh discloses a web tracking apparatus wherein a lateral movement of the web in a direction transverse to the direction of movement of the web along a path is monitored, and signals are generated in response thereto. In response to the signals, a determination is made whether an operation can be performed upon the web without imposing a correction to the lateral web movement. The device thus requires sensing of web movement in a plurality of directions.
U.S. Pat. No. 4,937,664 to Chiku et al discloses an image forming apparatus which eliminates a possible image misregistration in each image station. The apparatus detects register marks formed by each image forming station. When misregistration occurs, a correcting means begins operation at a predetermined time which is independent from the image forming sequence of each image forming station. The misregistration is corrected without causing a rate of operation of the apparatus to decrease. Accordingly, the apparatus requires the use of an additional correcting means.
U.S. Pat. No. 4,937,635 to Paxon et al discloses a reproduction apparatus which corrects registration errors. Reproductions of reproductions are enlarged and examined for image registration. Appropriate adjustments are made in the timing of process steps to provide a registration suitable for high quality image reproduction. This apparatus requires the production of additional reproductions for examination purposes, thus increasing the time required to produce an image.
While the related art attempts to improve color separation registration in a color printer, the art proposes no structure for achieving this goal which is effective for all subsystems of the printer without requiring additional timing control devices, perfect synchronization and/or perfectly designed components.